In general terms, electric subsea installations and devices usually demand high standards regarding durability, long-term functionality and independence during operation. Electric subsea installations that need to be cooled during operation, such as subsea converters, require an autonomous and durable cooling of its components. It is known to use a dielectric liquid of low compressibility such as for example mineral oil as a cooling fluid. The dielectric fluid could also be composed of natural or synthetic esters. In general terms, the dielectric fluid is used to provide a pressure compensated environment, and additionally functions as an electric insulation medium of electric components, such as capacitor units, placed in the electric installation.
The tanks of power-electronic subsea equipment, such as subsea converters, are thus typically filled with oil, which acts as a combined electric insulation and cooling medium. To improve the cooling for high-loss electric components such as power semiconductors, these electric components are typically attached to heat sinks, through which the oil flows. The oil receives heat from the internal converter components and may transfer it to the sea water through the tank surface. Generally, the tank surface alone is not sufficient for the cooling and the cooling capacity can be augmented by a sea-water heat exchanger located outside the tank. To improve the cooling for high-loss electric components such as power semiconductors, these electric components are typically attached to heat sinks, through which the oil flows.
Because of the high reliability requested for the subsea converters, it may be advantageous to have a passive cooling system for the subsea converter, i.e., a cooling system without the need for a pump. The oil in the tank and in any oil-to-sea-water heat exchanger can be moved by natural convection. Also the sea water cooling the oil-to-sea-water heat exchanger may be moved by natural convection. Ideally it may thus be desirable to move the oil by natural convection only, to eliminate the pump as a component that has a finite lifetime and that may fail. One issue with oil-natural-convection cooling is, however, the limited efficiency.
Generally, dielectric fluids such as mineral oil have high viscosity and low thermal conductivity. Furthermore, the natural convection flow rate of a typical dielectric fluid in a heat exchanger is generally in the laminar regime. Typical cooling systems for subsea power-electronic equipment are thus limited by the high thermal resistance between the cooling fluid and the wall necessitating a large heat-transfer surface. A sea-water heat exchanger may be preferably manufactured from circular pipes, since this is a semi-finished product form in which typical heat exchanger materials such as stainless steel are readily available. However, the pipes cannot have a too small inner diameter. Otherwise, due to the wall thickness needed, the steel cross-section would be large compared to the oil-flow cross-section, making the heat exchanger heavy and costly. Respecting a sufficient inner diameter, however, generally makes the heat transfer from oil to inner tube wall poor (due to laminar flow and low thermal conductivity of the oil).
To keep the pressure drop low enough for natural convection, there should be a sufficient oil-flow cross-section (e.g., many parallel pipes) and the pipes should not be too long. In particular, coiled-tube heat exchangers, enabling long tubes, cannot be used. Each tube must be connected to a manifold at both of its ends (e.g., by welding). Many tubes connected in parallel result in a large number of connections (typically welds). There may be in the order of thousand connections for large converters. Since providing the connections requires manual work, this means high cost, next to the challenge of keeping all connections tight.
US 20130056181 discloses a deionized-water cooling system for electrical equipment, connected electrically to a primary power supply. The system includes a main circuit to channel and cool the deionized water intended to circulate within the electrical equipment; a main pumping system; a main power source; a deionization circuit connected at two points to the main circuit and including a deionizer; a secondary pumping system to circulate the deionized water in the deionizer, and a secondary power source, which secondary power source has less power than the main power source. US 20120189472 discloses a hydraulic power unit for subsea use. The power unit includes a housing containing a fluid, an electric motor mounted in the housing, a distribution pump, a heat exchange unit provided externally to the housing and at least one distribution conduit in fluid communication with the heat exchange unit and the housing
While such systems may be thermally efficient, they require oil channels to be built around the components to cool, or the heat sinks to which they are attached. The oil in the channels is under overpressure with respect to the oil around the channels. So, the channel walls need to be strong and tight enough to sustain this overpressure. In addition, the channel walls must be thermally insulating to limit the heat transfer from high-loss components that may get hotter, to low-loss components that are more temperature sensitive. This significantly adds to the mechanical complexity of the system.
Consequently, the cooling efficiency is limited, and the design of an economical cooling system may thus be challenging. Hence there is a need for efficient subsea cooling of electric components in subsea applications.